Spectral filter and spectrometric device

Abstract

A spectral filter (10) is provided with a long-pass filter (12) and a short-pass filter (13). The long-pass filter (12) has a film thickness gradient GL wherein film thickness increases monotonically in a single direction, and transmits light of a wavelength region longer than a cut-off wavelength WL. The short-pass filter (13) has a film thickness gradient GS wherein film thickness increases monotonically in a single direction, and transmits light of a wavelength region shorter than a cut-off wavelength WS. The long-pass filter (12) and the short-pass filter (13) are overlapped such that the single directions match each other. At the positions in the single directions, a transmittance peak is formed by the cut-off wavelength WL being shorter than the cut-off wavelength WS. The film thickness gradient GL is greater than the film thickness gradient GS.

The invention claimed is: 
     
       1. A spectral filter comprising: a long-pass filter having a film thickness gradient GL such that film thickness increases monotonically in one direction, the long-pass filter transmitting light in a wavelength range longer than a cutoff wavelength WL, the cutoff wavelength WL lengthening monotonically as the film thickness increases; and a short-pass filter having a film thickness gradient GS such that film thickness increases monotonically in one direction, the short-pass filter transmitting light in a wavelength range shorter than a cutoff wavelength WS, the cutoff wavelength WS lengthening monotonically as the film thickness increases, wherein the long-pass filter and the short-pass filter are laid over each other so that the one direction of the long-pass filter is a same direction as the one direction of the short-pass filter, at any position in the one direction, the cutoff wavelength WL is shorter than the cutoff wavelength WS so as to form a peak in transmittance, and the film thickness gradient GL is larger than the film thickness gradient GS, and wherein when a halt-maximum width of the transmittance at a wavelength of 380 nm is represented by Δλ 380 , a half-maximum width of the transmittance at a wavelength of 780 nm is represented by Δλ 780  (nm), and Δλ 380 /Δλ 780  is represented by Δλ, then the spectral filter fulfills 0.3≤Δλ≤4. 
     
     
       2. The spectral filter according to  claim 1 , wherein the long-pass filter and the short-pass filter are each composed of a multi-layer film in which layers of a first-refractive-index material and layers of at least one second-refractive-index material with a refractive index higher than a refractive index of the first-refractive-index material are stacked together, and when, among the at least one second-refractive-index material, a material with at least either a largest number of layers or a largest total film thickness is taken as a main-refractive-index material, in the long-pass filter, refractive indices of the main-refractive-index material at the wavelengths of 380 nm and 780 nm are represented by nL 380  and nL 780  respectively, a film thickness of a transmission portion having a peak in transmittance at the wavelength of 780 nm is represented by dL 780 , and a film thickness of a transmission portion having a peak in transmittance at the wavelength of 380 nm is represented by dL 380 , and in the short-pass filter, refractive indices of the main-refractive-index material at the wavelengths of 380 nm and 780 nm are represented by nS 380  and nS 780  respectively, a film thickness of a transmission portion having a peak in transmittance at the wavelength of 780 nm is represented by dS 780 , and a film thickness of a transmission portion having a peak in transmittance at the wavelength of 380 nm is represented by dS 380 , then the spectral filter fulfills conditional formula below:
   0.99≤{( dL   780   /dL   380 )/( dS   780   /dS   380 )}×[{( nL   780   /nL   380 )+( nS   780   /nS   380 )}×½] 0.4 ≤1.065  (1).
 
 
     
     
       3. The spectral filter according to  claim 1 , wherein the spectral filter fulfills 0.4≤Δλ≤2. 
     
     
       4. The spectral filter according to  claim 3 , wherein the long-pass filter and the short-pass filter are each composed of a multi-layer film in which layers of a first-refractive-index material and layers of at least one second-refractive-index material with a refractive index higher than a refractive index of the first-refractive-index material are stacked together, and when,
 among the at least one second-refractive-index material, a material with at least either a largest number of layers or a largest total film thickness is taken as a main-refractive-index material, in the long-pass filter, 
 refractive indices of the main-refractive-index material at the wavelengths of 380 nm and 780 nm are represented by nL 380  and nL 780 , respectively, 
 a film thickness of a transmission portion having a peak in transmittance at the wavelength of 780 nm is represented by dL 780 , and 
 a film thickness of a transmission portion having a peak in transmittance at the wavelength of 380 nm is represented by dL 380 , and in the short-pass filter, 
 refractive indices of the main-refractive-index material at the wavelengths of 380 nm and 780 nm are represented by nS 380  and nS 780  respectively, 
 a film thickness of a transmission portion having a peak in transmittance at the wavelength of 780 nm is represented by dS 780 , and 
 a film thickness of a transmission portion having a peak in transmittance at the wavelength of 380 nm is represented by dS 380 , 
 then the spectral filter fulfills conditional formula (2) below:
   0.995≤{( dL   780   /dL   380 )/( dS   780   /dS   380 )}×[{( nL   780   /nL   380 )+( nS   780   /nS   380 )}×½] 0.4 ≤1.03  (2).
 
 
 
     
     
       5. The spectral filter according to  claim 1 , wherein
 the long-pass filter is deposited on one surface of a substrate, and 
 the short-pass filter is deposited on another surface of the substrate. 
 
     
     
       6. The spectral filter according to  claim 5 , wherein an angle between the one surface of the substrate and an outermost surface of the long-pass filter is represented by α and an angle between the another surface of the substrate and an outermost surface of the short-pass filter is represented by β, and wherein the film thickness gradient GL equals |tan α| and the film thickness gradient GS equals |tan β|. 
     
     
       7. A spectrometric device comprising:
 the spectral filter according to  claim 1 ; and 
 a plurality of photoreceptive elements which receive light transmitted through the spectral filter, 
 wherein the photoreceptive elements are arranged along the one direction in which the film thickness increases monotonically in the long-pass and short-pass filters of the spectral filter. 
 
     
     
       8. The spectral filter according to  claim 1 , wherein the long-pass filter and the short-pass filter are each composed of a multi-layer film in which layers of a first-refractive-index material and layers of at least one second-refractive-index material with a refractive index higher than a refractive index of the first-refractive-index material are stacked together.